Last December, her daughter Sue Nicholls filled her heating oil tank while she was in hospital for a hip operation.

Two days later, the oil had gone. Just over 1,000 litres had been taken, at a cost of about £800.

Ms Nicholls told X-Ray: "I wonder if these people will do it to their own mothers if they knew the situation that they'd left my mother in.

"And what would have happened if she was at home. Then the house would have been cold. So in one respect it was good that she was in the hospital, but in another respect not very good at all".

North Wales Police have carried out a large-scale investigation.

Pc Gary Aldous added: "The individuals who we believe are doing this kind of crime, they know what they are doing, they have got all the equipment ready and obviously they've got an industry and are selling it on.

"We've only actually caught somebody in the process of doing it on the one occasion.

"It's not something that's definitely happening of a night time either, we've got a suspicion on one of the other jobs we've been at, that it's been as early as seven o'clock in the evening."

I truly wish they could appreciate the distress that they are bringing to communities The Reverend John Jones

He added: "Having looked at the crime figures since November, I've had 32 thefts of oil, and over 12 village areas, which has had a massive impact on those communities."

The Reverend John Jones is responsible for churches at Nercwys and Treuddyn in Flintshire, which have been hit by heating oil thieves twice this winter

"It's been absolutely freezing at times. We've shortened services because the breath that was coming out my mouth at the altar was like icicles," he explained.

'Elderly and vulnerable'

"I truly wish they could appreciate the distress that they are bringing to communities and especially to the elderly and those who are vulnerable".

Despite the weather starting to warm up, and heating oil prices falling again, the thieves are not being put off.

Self-employed builder Ian Davies from Mold, who was targeted a few weeks ago, said: "It was about half past seven one night.

"I heard a little bit of knocking on the fence and didn't really pay much attention to it - until the next night when I turned my heating on and of course it didn't work," he recalled.

His tank had been drained dry of £200 worth of fuel.

He said he is now considering protecting it with a concrete wall and only fills it with 25 litres at a time.

"Cheeky people, they just don't care about anybody or anything. It's really low," he added.